I understand what ReallyTired is trying to say.
Also, agree with MMmomKK.
The first thing to do is get your facts right by talking with the teacher.
It could be that it is all as DD told you, or that it is not exactly like that and there is a misunderstanding regarding the tables and the other 4 children being the only ones who get extended or harder work.
But most importantly, adress with the teacher how your daughter is feeling, and how it is affecting her confidence. It could be that she is really good, and in the right table, but by comparing herself to the other children she feels she is not clever when that is not the case.
She also thinks that the teacher doesn't notice her, and the teacher might say that is not the case, but once you let her know about your daughter's feelings she'll be able to acknowledge them and make sure she knows that the teacher is noticing her (maybe focusing on more praise, etc).
Also, check with your daughter is she feels the work she is getting is right for her, or too easy and she feels frustrated at not being able to do harder work. When you discuss it with the teacher then you'll be able to see whether the teacher is right about the table/work she gets, or maybe your daughter is right and needs harder work.
All in all, the most important thing is that your daughter stops comparing herself or places too much importance on this to define whether she is clever or not. I know this is the most difficult part. But there will always be someone else who is faster, better than us at something, and that doesn't mean we are not good or clever enough.
Just speculating, it could well be your daughter is doing well and above average, right on target, but these other 4 children are working two years ahead of the curriculum and therefore need to be on a different table occasionally to do this type of work, and this does not mean that the rest of the class doing curriculum apropriate work are not clever.